Since potassium alum was employed in Ancient Egypt for clarification of water of surface water sources, there appeared a need for removal of the sludge staying in barrels after decantation of the settled water. However, the quantity of this sludge did not put forth acute problems before mankind.
Subsequently, the processes of water treatment began to employ slow filters wherein water was purified due to biological processes in the surface film, and regeneration of filters consisted in removal of a filtering layer 5 to 10 mm thick.
Since this method of regeneration did not require washing water, the problem of sludge disposal did not naturally arise.
With the advent of rapid filtration, a filtering charge entrapping various contaminants in the course of filtration has been regenerated by an upstream flow of water, which has raised the problem of treatment of washing water and in particular the problem of treatment and utilization of washing water sludge.
According to the present state of the art, a water treatment plant with an output of 1,000,000 cubic meters per 24 hours requires about 50,000 cubic meters of washing water per 24 hours.
This washing water contains about 2,500 cubic meters of the sludge.
Most water treatment plants in different countries discharge their washing water with the sludge therein into neighboring streams and lakes, frequently into those which serve as a source of water supply. Such a practice results in a serious pollution of the environment particularly when water treatment plants are located in cascade on large rivers and each downstream plant treats not only river water, but also a considerable amount of the sludge discharged by an upstream plant.
More rarely, washing water is settled and either again subjected to the process of treatment or, with an addition of a sufficient proportion of pure water, is employed for washing of filters.
At present, rather a large number of methods of treatment of the washing water sludge is known to those skilled in the art.
As a rule, the washing water sludge is subjected to thickening by means of additional settling for a few days, filtering through drum vacuum filters or centrifuging.
However, a high specific resistance of the sludge hinders its dewatering and hence thickening, which makes its filtering and centrifuging inefficient.
For the same reason many attempts to prevent the pollution of the environment by washing water sludge involve the reduction of a sludge volume by means of thermal drying or freezing out thereof.
Freezing out of the sludge as well as its thermal drying makes it possible to reduce the sludge volume scores of times, its concentration being simultaneously increased.
In particular, there is known in the prior art a method of treatment of washing water sludge (cf. U.S. Pat. No. 3,720,608), consisting in dewatering of the sludge by means of thermal drying.
Also known in the art is a method of treatment of washing water sludge (cf. U.S. Journal "Water and Sewage Works", v. 112, No. 11, 1965, pp. 401-406), consisting in dewatering of the sludge by means of thermal drying which is preceded by freezing and thawing of the sludge, intended to destroy its structure and thus to increase the efficiency of its dewatering.
Besides, there is known in the art a method of sludge treatment (cf. U.S. Journal "AWWA", v. 61, No. 10, 1966, pp. 541-566), consisting in dewatering of the sludge by freezing out thereof whereupon it is subjected to vacuum filtering.
All the methods of sludge treatment mentioned hereinabove enable the sludge volume to be reduced scores of times, its concentration being simultaneously increased. However, all these methods are rather expensive since they require special equipment and considerable consumption of energy. Besides, they do not solve the problems of pollution abatement since the sludge dewatered by any of these methods is always removed to a dump where it is dissolved by rain-water, melted snow, and floods and brought back into surface lakes and streams and ground water horizons.
Thus, though the cost of sludge treatment has risen considerably and sometimes exceeds that of potable water treatment, the present state of the art does not prevent the pollution of the environment by the washing water sludge which is not utilized in any way.
In this respect only utilization of washing water or sludge contained therein makes it possible to solve the problem of pollution abatement and to bring it into agreement with the requirements of maintaining an ecological balance.
Though the problem of preventing the pollution of the environment by washing water sludge has long been in existence, it has not been solved up to the present since there are no methods for utilization of this sludge.